New Mexico Geological Society Annual Spring Meeting & Ft. Stanton Cave Conference — Abstracts

Gravity surveys are an effective way to aid in interpretation of subsurface bedrock variations in sedimentary basins. A gravity survey was conducted in the San Marcial basin, Socorro County, New Mexico over the summer of 2021 in order to clarify general basin geometry and structural complexities that could not otherwise be discerned by field mapping. The San Marcial basin was formed as part of the Rio Grande rift, which produced several extensional sedimentary basins from southern Colorado through New Mexico. In this study, approximately 100 new gravity measurements were tied to National Geospatial-Intelligence Agency (NGA) benchmarks. At each location, gravity and GPS measurements were taken, with GPS coordinates measured to a resolution of 10cm horizontal and 5cm vertical. Reviewing the processed data indicated two significant, linear-trending, terrain-corrected Bouguer gravity anomalies in the south-central and eastern part of the basin. The western anomaly is correlated to the down-to-the-east Black Hill fault, which strikes NW and forms fault scarps on select middle Pleistocene surfaces. The eastern anomaly is a pronounced west-down gravity gradient that strikes NNE, which we interpret as a west-down fault. This eastern anomaly, here named the Lava fault, coincides with a laid-back escarpment east of the Rio Grande. The Lava fault projects southwards to 5 km east of the northern end of the Fra Cristobal Mountains. To the north, it may possibly link with the Little San Pascual fault system immediately east of Mesa del Contadero. The lack of noteworthy footwall uplifts along either fault, especially when compared to the prominent Fra Cristobal Mountain footwall uplift on the Walnut Springs fault to the south, may be due to a northward partitioning of extensional strain from the Walnut Springs fault towards the Black Hill and Lava faults. The decreased displacement on either fault resulted in low vertical displacement rates that could not outpace long-term erosion rates or burial by sedimentation.